object JSON serialization that's circular-ref safe

gistfile1.js

// all this `toJSON()` does is filter out any circular refs. all other values/refs,
// it passes through untouched, so it should be totally safe. see the test examples.

Object.prototype.toJSON = function() {
	function findCircularRef(obj) {
		for (var i=0; i<refs.length; i++) {
			if (refs[i] === obj) return true;
		}
		return false;
	}

	function traverse(obj) {
		function element(el) {
			if (typeof el === "object") {
				if (el !== null) {
					if (Date === el.constructor || Number === el.constructor || Boolean === el.constructor || String === el.constructor || RegExp === el.constructor) {
						return el;
					}
					else if (!findCircularRef(el)) {
						return traverse(el);
					}
				}
				return null;
			}
			return el;
		}

		var idx, tmp, tmp2;

		if (Object.prototype.toString.call(obj) === "[object Array]") {
			refs.push(obj);
			tmp = [];
			for (idx=0; idx<obj.length; idx++) {
				tmp.push(element(obj[idx]));
			}
			refs.pop();
			return tmp;  
		}
		else if (typeof obj === "object") {
			if (obj !== null) {
				if (Date === obj.constructor || Number === obj.constructor || String === obj.constructor || Boolean === obj.constructor || RegExp === obj.constructor) {
					return obj;
				}
				else if (!findCircularRef(obj)) {
					refs.push(obj);
					tmp = {};
					for (idx in obj) { if (obj.hasOwnProperty(idx)) {
						tmp2 = element(obj[idx]);
						if (tmp2 !== null) tmp[idx] = tmp2; 
					}}
					refs.pop();
					return tmp;
				}
			}
			return null;
		}
		else return obj;
	}

	var refs = [], ret;
	ret = traverse(this);
	refs = [];
	return ret;
};

test.js

var a = {
   b: 12,
   c: true,
   d: "foobar",
   e: {
      f: function() { alert("blah"); }, // functions get ignored
      g: new Date(), // dates get their own `toJSON()` serialization called
      h: [ true,1.3,"haha" ]
   },
   k: {},
   l: /foobar/g // regexes get turned into an empty {}
};
a.i = a; // circular ref!!
a.e.i = a; // circular ref!!
a.e.j = a.e; // circular ref!!
a.e.h.push(a.e); // circular ref!! since it's in array, will be replaced with `null`
a.k.m = a.e; // **NOT** a circular ref, just a dupe ref, so leave it alone!!

// Look Ma! No circular refs!
JSON.stringify(a); // {"b":12,"c":true,"d":"foobar","e":{"g":"2012-08-17T12:11:05.647Z","h":[true,1.3,"haha",null]},"k":{"m":{"g":"2012-08-17T12:11:05.647Z","h":[true,1.3,"haha",null]}},"l":{}}